In modern ophthalmology there is a growing demand for accurate images of the eye and of the retina, in particular, to allow practitioners to inspect and diagnose various medical conditions involving the fundus (retina) and to facilitate advanced treatments. Effective fundus imaging should clearly show the features of the fundus and any artifacts on its surface, and should be at least consistent with the actual fundus.
Fluorescein angiography is an extremely valuable test that provides information about the circulation system and the condition of the back of the eye. Fluorescein angiography tests are useful in evaluating many eye diseases that affect the retina and the choroid (sub-retina). The test is performed by injecting a special orange-colored dye, called fluorescein, into a vein in the patient's arm, from which the dye circulates through the blood vessels of the body. As the dye enters the blood vessels of the eye, a series of photographs of the retina are taken with a camera equipped with special filters. These filters highlight the fluorescein dye and make it stand out against the background of the retina. The series of photographs is then replayed in a sequence in a movie loop, so that the ophthalmologist may follow the fluorescein dye advance through the blood vessels. Any circulation problems, swelling, leaking or abnormal blood vessels on the retina or the choroids will be revealed by the dye and its patterns.
A significant clinical challenge relates to the normal patient movement during the series of images that may be taken over a period of time from 1 minute to 15 minutes or longer. A solution that allows the same relative location of specific pathologies within the imaged area is valuable from a clinical perspective, as it allows the stabilization of these pathologies on the screen and a simpler assessment of the growth, or the change in location of hyper-fluorescent, or hypo-fluorescent areas of the retina/choroid. The ophthalmologist can then take a determination as to the diagnosis and possible treatment options; now with higher efficacy with the pathology now stabilized as to its relative location in the series.
As is well known, the fundus is a substantially spherical surface. For purposes of capturing a spherical surface on an image, a wide angle lens may appear to be a good choice. Unfortunately, wide angled lenses are characterized in that the wider the angle of view of the lens the lower the resolution of the images produced using the lens will become. Furthermore, the translation of the image captured by the wide angle lens to a planar image creates some distortions at the edges of the image. The distortions commonly increase with the angle of view of the lens. These distortions are commonly referred to as “wide-angle distortion”. Additionally, it is quite problematic to provide adequate illumination of a wide angle of the fundus.
Thus, the lenses used for photographing the fundus are typically not wide angle lenses and have a substantially smaller field of view. Commonly used retinal (fundus) cameras typically provide an effective view of about 30-60 degrees of the retina per photograph (or image) taken. As a result, in order to provide a representation of larger portions of the retina, several images are taken from different angles. In order to ease the diagnosis and storage of the retinal representation, the images need to be stitched to one another to provide a single synthetic montage image or mosaic which covers the required field of view.
In order to achieve a consistent planar representation of the fundus (or of any significant portion of the fundus) the images need to be correctly matched and positioned in relation to one another. In the past, matching of the images was done manually. Matching of images manually requires considerable skill and time and is sometimes not accurate. Although attempts have been made to provide a method of automatically matching of images, these methods provide a local matching of pairs of images only, and even if one succeeds in locally matching pairs of images, simply having matching pairs is not enough to provide a consistent planar representation of the fundus.
There is thus a need to provide a system and a method of enabling automatic production of a consistent planar representation of the fundus from a plurality of images, as part of which, one or more groups of matching images comprised of more than two images are locally and globally matched.
Furthermore, the current techniques used for creating a planar representation of the fundus are inherently limited. In order to represent any significant portion of a spherical surface on a plane, some areas of the spherical surface must be modified (squeezed and stretched). Thus, even if one succeeds in creating a consistent planar representation of the fundus, the planar representation of the originally spherical surface cannot be accurate.
There is thus a further need to provide a system and a method of creating a consistent and substantially accurate representation of the fundus. There is yet a further need to provide system and method of creating a consistent and substantially accurate spherical montage representing the fundus.